This invention relates in general to scanned display systems and in particular to those utilizing cathode ray tubes such as television receivers.
Among the geometric distortions produced when a beam emanating from a theoretical point source is caused, by synchronous deflection along two axes, to scan a viewing screen having a radius of curvature greater than the center screen-to-point source distance, is pincushion. Pincushion is readily observable when information in the form of a graticule is displayed appearing as a "bowing in" of the reproduced image. If the viewing screen has a greater radius of curvature along both axes (as is the case in present day television receivers), pincushion results along both axes, that is, along horizontal and vertical lines. The former is called "top and bottom" pincushion and the latter "side" pincushion. Quite obviously pincushion distortion is maximized when the radius of the screen curvature is infinite (i.e., flat screen), and while cathode ray tubes generally do not have flat screens, aesthetic considerations usually dictate that nearly flat screens be used.
Of particular interest in the present invention is the top and bottom pincushion or "vertical sag" as it is often called. Analysis of the scanning process typically used in cathode ray display systems shows that a high frequency horizontal deflection system causes side-to-side scanning of the viewing screen while a lower frequency vertical deflection system causes successive side-to-side scans to progress in a downward direction.
It is well known that correction of top and bottom pincushion distortion may be accomplished by adding an appropriate horizontal frequency deflection component to the normal vertical deflection signal. It is also well known that top and bottom pincushion is zero for the midscreen horizontal scan line and increases progressively (i.e., greater sagging) with increased vertical deflection angle. As a result, the correction signal is required to vary in amplitude from a maximum at one polarity, corresponding to horizontal scan lines at the top of the viewing screen through zero, corresponding to those at midscreen, to a maximum at the opposite polarity corresponding to the bottom of the screen. In addition to the amplitude variations described, the horizontal rate signal should, for ideal correction, be of such character that its effect upon the vertical deflection is the complement of the distortion. Such a waveform is quite complex and difficult to fabricate and general practice is to approximate the ideal correction in the form of either a cosine, parabolic, or sine squared waveform.
Top and bottom pincushion compensation systems may be categorized as being either high level or low level, the former characterized by direct yoke current correction and the latter by addition of a correction signal to the vertical deflection amplifier. High level correction uses a saturable reactor in series with the vertical deflection yoke. Horizontal rate signals are applied to balanced inputs of the reactor and a sample of the vertical deflection signal is also applied. The reactor is wound such that the amplitude and polarity of the induced horizontal component coupled to the vertical deflection yoke is determined by the instantaneous polarity and amplitude of the applied vertical signal.
A major drawback of saturable reactor correction is that the reactor must carry the entire vertical scanning current. Conventional cathode ray display systems, such as those used in television receivers, employ "saddle" wound yokes which exhibit relatively high impedances and, therefore, require low deflection currents. As such they produce an ideal environment for the saturable reactor correction systems. However, the present trend is toward the use of toroidal yokes which are characteristically lower in impedance and, therefore, require a greater deflection current to scan corresponding cathode ray tubes than do saddle yokes.
With the increased deflection current required by toroidal yokes, the saturable reactor becomes a prohibitively large, wasteful and expensive device and a very inefficient mechanism for correcting pincushion distortion. As a result the alternative low level systems are currently enjoying increased attention from display system manufacturers.
As mentioned, low level correction systems differ from the above-described high level system in that the appropriate correction waveform is produced by specialized circuitry at a relatively small amplitude and applied to an appropriate point in the vertical deflection amplifier configuration rather than directly to the "high power" yoke.
One of the currently used low level systems comprises a complex circuit in which a balanced modulator imposes the horizontal deflection component on a vertical scan waveform. The circuitry used is complex requiring a two phase, or balanced, output and switching circuitry to switch from one output to another to accomplish the required midscreen phase reversal. This system requires a large number of active devices. Another low level scheme currently used includes a horizontal signal amplifier driven by a horizontal rate sine wave signal. An additional phase inverter is switched into the circuit alternating the polarity of the signal applied to the vertical deflection amplifier during the appropriate portion of vertical scan. This system, while less costly than the modulator scheme, does not appear to consistently produce the required correction signal needed and finds little use.